Masao Norita

The corticostriatal and corticotectal projections of the feline lateral suprasylvian cortex demonstrated with anterograde biocytin and retrograde fluorescent techniques

The relationship between the visual cortex and the striatum (ST) of the cat is poorly understood. The present experiments were an attempt to determine if regions along the lateral suprasylvian cortex (LS), known to send dense visual projections to the superior colliculus (SC), also project to the striatum and, if so, to determine whether corticostriatal and corticotectal axons arise from the same neurons. Injections of the anterograde tracer, biocytin, into the posterior portion of the lateral suprasylvian cortex resulted in dense label in both ST and SC. In ST, labeled fibers and terminals were found predominantly in the caudal part of the head of the ipsilateral caudate nucleus and the caudal portion of the ipsilateral putamen. These injections also resulted in label in the superficial and deep laminae of SC. After paired injections of retrogradely transported fluorescent dyes (dextran tetramethylrhodamine and dextran fluorescein) into ST and SC, numerous labeled LS neurons were observed in layer V and modest numbers in layer III: the corticostriatal neurons were found in layers III and V whereas corticotectal neurons were seen only in layer V. Although labeled neurons from each injection were intermingled in layer V, very few of them were double-labeled. These data suggest that while ST and SC receive substantial visual inputs from the same cortical area, the nature of the information they receive may be quite different.

Early development of the oligodendrocyte in the embryonic chick metencephalon

It has been demonstrated that the spinal cord oligodendrocytes in the vertebrates arise in the ventral ventricular zone adjacent to the floor plate in their early development. Because of the similarities of basic structures in the spinal cord and metencephalon, it is probable that the mode of early oligodendrocyte development in the metencephalon is the same as that in the spinal cord. We examined this possibility in chick embryos, using monoclonal antibodies O1 and O4, markers for oligodendrocyte lineage. An O4‐positive (O4+) cell focus was observed in the medial ventricular zone of E5 chick ventral metencephalon (the earliest stage examined), adjacent to the floor plate. At E6, O4+ cells were dispersed from the medial to the lateral pons and, at E7, to the cerebellar anlagen. O4+ cells in the E6 brainstem and in the E7 cerebellum were unipolar in shape, whereas one day later, some of the labeled cells were multipolar with a few thin processes. O1 + oligodendrocytes first appeared at E8 in the ventromedial part of the pons and were distributed throughout the pons at E10 and in the cerebellum at E12. Explants from three subdivisions of the metencephalon (medial and lateral pons, and cerebellum) from E5 to E8 chick embryos were separately cultured to confirm the potential for generation of oligodendrocyte lineage. O4+ cells appeared in the culture of the E5 medial pons (the earliest stage examined), in the E6 lateral pons, and in the E7 cerebellum. In addition, E7 was the youngest stage from which cerebellar explants were able to generate O1+ oligodendrocytes. Our results clearly demonstrated the in vivo morphology of oligodendrocyte precursors in the metencephalon and their developmental appearance in a ventral‐to‐dorsal manner. From the bipolar morphology of O4+ cells and the spacio‐temporal continuity of the dispersion, it is inferred that the initial dispersion of O4+ cells may involve oligodendrocyte migration from the focus of the medial pons to the lateral and dorsal parts of the metencephalon. J. Neurosci. Res. 48:212–225, 1997.

Multisensory responses and receptive field properties of neurons in the substantia nigra and in the caudate nucleus

The basal ganglia are widely regarded as structures involved in sensorimotor co‐ordination, but little is known about the sensory background of their function. We publish here descriptions of the excitatory sensory responses and receptive field properties of the visual, auditory, somatosensory and multisensory caudate nucleus and substantia nigra pars reticulata neurons. Altogether 111 caudate nucleus and 124 substantia nigra sensory neurons were recorded in halothane‐anaesthetized, immobilized, artificially ventilated cats. The sensory properties of the caudate and nigral neurons were found to be quite similar. A majority of the units were unimodal while a significant proportion of them were multisensory. The visual and the somatosensory modalities predominated for both nuclei. The sensory receptive fields were extremely large. The visual and auditory receptive fields covered the whole physically approachable sensory field and the somatosensory receptive fields covered the whole body surface of the animal. The receptive field properties of the multisensory caudate and nigral units were similar to those of the unimodal neurons. We observed no signs of retinotopical or somatotopical organization within the basal ganglia. The particular sensory receptive field properties, together with the significant number of multisensory neurons in the basal ganglia, suggest the existence of a multisensory pathway of presumably tectal origin involving the caudate nucleus and the substantia nigra that may serve for the sensory feedback of motor actions co‐ordinated by the basal ganglia.

Remodeling of reciprocal synapses associated with persistence of long-term memory

We investigated a model of long‐term memory in which the female mouse establishes pheromonal memory of its partner at mating. We examined the reciprocal synapses of the accessory olfactory bulb and found that pheromonal memory was associated with morphological changes in excitatory synapses in the early phase of memory acquisition and by changes in inhibitory synapses in the late phases of memory persistence. After extinction of pheromonal memory, these morphological changes were no longer present. These findings suggest that the persistence of pheromonal memory is associated with continuous and dynamic changes in the morphological plasticity of reciprocal synapses in the accessory olfactory bulb.

A comparative immunocytochemical study of development and regeneration of chemosensory neurons in the rat vomeronasal system

Vomeronasal neurons undergo continuous neurogenesis during development and after neuronal injury. We used immunocytochemical methods to compare different stages of the vomeronasal organ development to those of regeneration following vomeronasal nerve transection. At E15 and at 6 to 10 days after injury, nestin-positive cells were observed throughout the sensory epithelium. We did not find nestin immunoreactivity to be localized to the boundary region of the epithelium. The early appearance and wide distribution of nestin-positive cells suggests that they represent chemosensory precursor cells that develop and migrate vertically in the epithelium. Vomeronasal receptor cells degenerated 6 to 8 days after nerve transection, but axon terminals in the accessory olfactory bulb (AOB) continued to show the presence of the chemosensory specific marker (OMP) for up to ten days, a significant finding observed in this study. It is likely that the distance from the site of nerve transection may contribute to differences in the time course of anterograde and retrograde axon degradation. OMP-positive neurons were observed in the normal adult epithelium and to a much lesser extent 10-60 days after recovery from nerve transection. Axons from regenerated receptor cells did not reach the AOB during this time period. This failure to reestablish connections with target cells in the AOB could explain why OMP-positive cells were rarely observed among the regenerated cells in the vomeronasal epithelium.

Visual receptive field properties of neurons in the caudate nucleus

Visual single‐unit activity was recorded in the caudate nucleus of halothane‐anaesthetized, immobilized, artificially respirated cats. Visually sensitive neurons were found in the dorsolateral part of the caudate body. A majority of the units responded optimally to small spot‐like stimuli moving with velocities between 30 and 120 °/s. The receptive field of these units is large: it covers a major part of both the contra‐ and ipsilateral visual hemifields. No signs of retinotopy were observed. Most of the neurons display directional selectivity and are narrowly tuned to the direction of the moving stimulus. These physiological properties are consistent with recent morphological results that reveal multiple connections of the caudate nucleus with the superior colliculus through tecto‐extrageniculo‐thalamic pathways in the mammalian brain.

Chapter 17 Extrinsic and intrinsic connections of the cat's lateral suprasylvian visual area

The lateral suprasylvian visual area (LS) is known to have numerous interconnections with visual cortical areas as well as with subcortical structures implicated in visually-guided behaviors. In contrast, little data is available regarding connections within the LS itself. In order to obtain information about intra-areal connections and to re-investigate LS connectivity with various cortical and subcortical areas, the traces (biocytin or WGA-HRP) was injected into various loci along the medial and lateral banks of the LS. The anterograde tracer, biocytin injections into both medial and lateral bank produced label contained within the respective bank that extended rostrally and caudally from the infection site. In addition, following medical bank injections, considerable label was distributed throughout the fundus and, to a lesser extent, in the lateral bank. In contrast, no label could be detected in the medial bank after lateral bank injections, and, although label was observed in the fundus, it was restricted to the most lateral aspects. Moderate labeling could be observed in the medial bank following the tracer injection into the most rostral aspect of the lateral bank. It is likely that input derived from various visual cortical areas which project to the medial bank of the LS has access to this intra-areal circuitry. This may provide a route by which visual cortical information can be relayed to other cortical and subcortical structures involved in visually-guided behaviors such as the anterior ectosylvian visual cortex, striatum, and the deep layers of the superior colliculus, despite the fact that these structures themselves do not receive substantial direct projections from the visual cortical areas that are associated with the medial bank. Examination of the laminar location of the cells-of-origin of striate and extrastriate projections to LS using retrograde trace, WGA-HRP, revealed that the supragranular laminae of areas 17, 18 and 19 were the source of LS afferents whereas afferents from the other cortical areas (e.g., 20a, 20b, 21a, 21b, 7 and anterior ectosylvian visual area) were from both supra- and infragranular laminae. In addition, all LS subregions received intra-areal afferent projections from all LS cortical laminae. Thus, although rather clear hierarchical relationship between LS and visual cortical areas appears to exist, the interconnections among LS subregions provide no clear evidence of simple hierarchical relationships between regions LS or may have feed-forward and feed-back pathways.

Chapter 13 Corticotectal relationships: direct and “indirect” corticotectal pathways

Publisher Summary The chapter explains direct corticotectal projections to understand the anatomical substrate and the physiological mechanisms by which they interact. The lateral suprasylvian (LS) cortex is a primary source of direct visual cortical afferents to the deep laminae of the superior colliculus (SC). The observation that visual neurons become unresponsive or unselective after LS removal is interpreted as being consequence of losing direct corticotectal excitation. There are anatomical data consistent with an indirect corticotectal route via the striatum and substantia nigra, an alternative pathway by which the visual cortical activity can access the deep aminae of the superior colliculus. Recent experiments described in the chapter provide anatomical and physiological evidence to support this hypothesis and are consistent with the idea that, in addition to a direct influence on deep lamina neurons, LS may modulate tecto spinal neurons indirectly via its projection to the striaturn.

Cortical and tectal afferent terminals in the suprageniculate nucleus of the cat

The suprageniculate nucleus (Sg) of the cat was observed electron microscopically after wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injection into the anterior ectosylvian visual cortical area (AEV) and superior colliculus (SC). Small axon terminals filled with round synaptic (RS) vesicles were labeled with HRP injected into the AEV and SC, whereas large axon terminals containing round synaptic (RL) vesicles were labeled after HRP injection in the SC. After producing a lesion in the AEV and adjoining orbito-insular cortex and injecting WGA-HRP into the SC, degenerated terminals and HRP-labeled RS terminals were occasionally found to make synaptic contacts with a single dendritic profile of Sg neurons.

Anisotropic functional connections between the auditory cortex and area 18a in rat cerebral slices

We developed a new method to visualize the myeloarchitecture in fresh slices, and investigated the properties of the functional neural connections around the boundary between the primary auditory cortex (area 41) and area 18a in rat cerebral slices. A fresh slice illuminated by near-vertical light was observed with a CCD camera. The translucent images of the slice showed contrast patterns very similar to myeloarchitecture. The boundary between these areas was identified by the well-developed layer IV/V in area 41 but not in area 18a. Antidromic/presynaptic components of the field potentials stimulated and recorded across the areal boundary showed symmetric distribution, while the postsynaptic field potentials in the direction from area 41 to 18a were more prominent than those in the opposite direction in layer II/III. In contrast, the dominant direction of propagation of postsynaptic potentials was from area 18a to 41 in layer V. In the presence of 1 microM bicuculline, an inhibitor of GABA(A) receptors, the polysynaptic activities propagating from area 18a into 41 via layer V were elicited by stimulation of area 18a. The propagation measured by Ca(2+) imaging or field potential recordings was potentiated after both areas 18a and 41 were alternately stimulated several times.